(a) In the outward-facing state (top), the Zone two from the transport domain interacts with the scaffold domain, with the Zone one partially occupied by the ECL2a. Two different poses of the ECL2a were observed (shown in red and dark gray lines): one is situated at the side of the transport domain (red line); the other is crossing over the crests of ECL4 and ECL5 (dark gray line). In the inward-facing state (bottom), the ECL2a has to be repositioned, releasing the Zone one to interact with the scaffold domain. Residues from Zone one and Zone two are highlighted (Figure 1—figure supplement 5). (b) The inherent plasticity around the TM2-3 loop, ECL3, TM3, and TM6 regions (highlighted in dash lines) allows the transitions between the outward-facing (top) and inward-facing states (bottom). The relative position of transport domain in the outward-facing state is highlighted in dash (bottom panel in b). The regions of SLC1A5 are colored based on the Figure 1a. The light gray line depicts membrane boundary.